Powder-spray cabins are widely used for protecting the environment during the intense spraying of objects with powders, generally synthetic resin powders, serving to coat, color or treat an article, where the powder is entrained in a gas stream.
In general, the powder-spray cabin may have top and bottom walls and, extending around the spray chamber, peripheral walls which can include a pair of mutually parallel side walls and a pair of mutually parallel end walls.
Means can be provided for supporting the article in the chamber, e.g. for suspending the article therein, so that the article is contacted by the powder which can be sprayed from spray guns or nozzles through an inlet or spraying opening along the peripheral wall, e.g. in one of the side walls.
A suction path is formed from the chamber to an outlet opening, generally in an opposite side wall of the spray cabin which can lead to a suction source, e.g. a blower, and to a system for separating the excess particles from the entraining gas so that, for example, the particles may be reused.
It may be noted that the articles to be sprayed can be introduced through an opening or door in one end wall, can be suspended on a track extending through the enclosure and can be removed, as a coated article through an opening or door in the opposite end wall. The articles can be stationary within the enclosure during the spraying process or can continuously move therethrough.
Means can be provided to impart an electrostatic charge to the particles which causes them to adhere to the article which generally is at an opposite electrostatic charge.
In general, one seeks to maintain the spray-powder flow through the enclosure in approximately parallel streams which are laminar and to avoid turbulence and the forms of eddy currents within the enclosure. This not only is desirable for uniform coating of the workpieces since a laminar flow is most conducive to such uniform coating, but also reduces the tendency of the particles to settle out of the entraining gas. For this reason, attempts have been made to ensure that by and large all paths through the enclosure avoid turbulence sources and thus minimize changes of direction and speed of the gas to maintain the uniformity of flow.
It is possible to ensure that relatively smooth and uniform flow of gas through the enclosure and to minimize directional change and speed change when the powder collector or suction outlet is located in a flow direction directly behind the spray chamber. However, Where there is not a direct or straight path between the spray chamber and the suction duct or powder collector or for other reasons it is not possible to ensure a straight line flow of the gases containing the powder through the chamber, it can scarcely be avoided that a substantial quantity of the spray powder will deposit on the floor or bottom wall of the chamber and will not, therefore, be deposited on the articles to be coated. Deposited powder, of course, must be removed from the floor or bottom of the powder-spray chamber.
Significant problems arise when the raw gas is diverted from the predetermined straight line path through the cabin. This applies mainly to powder-spray cabins in which gas is drawn out horizontally and thus to double cabins. Indeed, when the outlet opening is high on a cabin wall, in edge regions deflection of the gas stream can cause flow turbulence which causes the powder in part to escape from the cabin and in part to deposit upon the bottom wall of the spray cabin.
Such deposition can reduce recirculating powder flow and may result in the introduction of impurities into the spray powder, partial mixture of the spray powder with powders of other characteristics and a coloration of the powder or a change in the color tone of the powder from the desired color tone.
A powder spray cabin of the above-described type is disclosed in British patent 1,315,671. In this case, between the side walls, a tray-shaped bottom is provided in addition to a vertical rear wall formed by a common flat baffle. The baffle begins at the lower edge of a spray inlet opening, rises and from above is deflected into a displaceable carriage provided with a filter arrangement whereby the powder is separated from the gas.
At the cabin side, the front wall of this baffle has a rectangular opening ahead of which, extending on all sides beyond this opening, is a shield or curtain which in the wall plane has a suction slit in an annular pattern surrounding the opening and through which raw gas is directly drawn by suction from the cabin into the duct.
The suction intake cross section under the underside of the window is greater than the slit cross section between the shield and the inner cabin wall. The raw gas which is drawn therethrough has a correspondingly larger flow rate.